This invention relates generally to piezoelectric crystal resonators and more particularly to the configuration of an AT-cut resonator blank for minimizing the force-frequency effect when mounted in and operated in a resonator housing.
There are several different conventional piezoelectric crystals, primarily in the form of quartz resonators, used to provide accurate frequency determination, stability and precise timing. Increased use of the limited radio spectrum and the expanding mobile applications of electronic communications equipment, including sensor applications, serve to place greater demands on both the accuracy of frequency control and the magnitude of mechanical forces incident on such equipment.
Presently, almost all precise quartz resonators operate using bulk acoustic waves of the thickness shear variety. Among them is the well known singly rotated AT-cut. These thickness shear crystals or thickness mode cuts, as they are frequently called, are normally in the form of thin, circular discs, a typical example of which is shown in U.S. Pat. No. 3,931,388, entitled, "Crystal Resonator Housing Configurations", issued to Erich Hafner, et al. on Jan. 6, 1976. In such devices, the mechanical contact area of the mounting clips are purposely kept small on the edge of the crystal plate to minimize the force-frequency effect. This effect involves the relationship between stresses due to the mounting support supplied to the crystal resonator and changes in resonant frequency upon being energized. Unfortunately, the mounting geometry used in conventional crystal resonators has the capacity of translating any forces communicated between the quartz plate and the mounting supports to high stresses within the relatively small mechanical contact area which affects the operational frequency of the device. Greatly increased sensitivity due to damage of the quartz vibrator plate has been known to occur for this arrangement, particularly where shock and acceleration are part of the environment.
A variety of prior art approaches have been used to mitigate this persistent problem. For example, four points of fixation, instead of two, on the crystal resonator forming preselected mounting angles have been used. Another approach is to use double rotated cuts. However, all known arrangements with any degree of force-frequency insensitivity utilize circular plates peripherally mounted at points of limited area which inherently produces a concentration of stress at each mounting point.
It is an object of the present invention, therefore, to provide a crystal resonator which is shaped and mounted in such a way as to minimize the force-frequency effect.
A further object of the present invention is to provide a crystal which is easily mounted and properly aligned with respect to a predetermined crystallographic axis.
It is yet a further object of the present invention to provide a crystal which is particularly adapted for mounting in a ceramic resonator housing.